The cholecystokinin B receptor (also known as CCK2R, CCKBR, and the gastrin receptor) is a trans-membrane, G protein-coupled receptor. CCK2R is primarily expressed in (i) the brain and central nervous system, and (ii) the mucosa of the gastrointestinal tract, including parietal and ECL cells. It is reported to be most abundantly expressed in the cerebral cortex of the mammalian brain where it has been implicated in the regulation of memory/learning and response to stress1. In the alimentary canal, its presence has been detected in the GI tissues, as well as exocrine and endocrine pancreas. Conflicting reports suggest the receptor is also expressed in peripheral tissues such as adipocytes and pancreas2-9, along with CCK1R, a G protein-coupled receptor with 50% homology to CCK2R. The natural ligands of CCK2R are gastrin and cholecystokinin peptides1.
A number of different types of human tumors have been found that overexpress or ectopically express CCK2R in high densities or at high frequencies including medullary thyroid cancers, insulinomas, small cell lung cancers, non small cell lung cancers, bronchial and ileal carcinoids, GIST tumors, and colon cancers, hepatocellular carcinomas, and pancreatic cancers10-18. One manner in which disregulation of CCK2R signaling may contribute to tumor formation or growth relates to gastrin signaling through the receptor. Gastrin is a peptide hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach upon binding of CCK2R. Gastrin has been reported to be an inhibitor of cancer cell apoptosis, likely through its ability to induce activation of the serine/threonine protein kinase PKB/Akt19-21. Several reports have shown that inhibiting the CCK2R receptor and the gastrin autocrine loop induces apoptosis and inhibits the proliferation of the cancer cell lines in vivo.
CCK2 receptors found in tumor tissue have been reported to include both normal protein as well as a constitutively active a CCK2R-receptor splice variant (CCK2i4svR) that has the fourth intron inappropriately retained, resulting in the addition of 69 amino acids in the third intracellular loop domain of the receptor, the domain known to be important for signal transduction24-29.
Cytotoxic agents that target CCK2R might serve to block uncontrolled activation of the receptor in tumors. Helpfully, because CCK2R is expressed in normal brain tissue, the blood brain barrier will block polar compounds that could affect normal activity of the receptor in the brain. Therefore, where CCK2R and its splice variant are expressed outside of the brain by a tumor, the tumor will be the only tissue targeted by CCK2R and CCK2i4svR-specific cytotoxic agents.
CCK2R-specific antagonists have been developed and extensive structure-activity relationships have reported. Both in vitro and in vivo studies have demonstrated that the growth potentiating effects of gastrin can be abolished in the presence of selective CCK2R antagonists. One such antagonist is Z-360, an orally-active CCK2R antagonist (Zeria Pharmaceuticals Co., Ltd., Tokyo, Japan) that has a Ki=0.47 nmol/L with a selectivity ratio over CCK1R=672. Preclinical studies have shown that oral administration of Z-360 along with the chemotherapeutic gemcitabine significantly inhibited subcutaneous PANC-1 tumor growth compared with either agent alone (27.1% inhibition) and significantly increased survival compared with the vehicle. This antagonist is currently in Phase II human clinical trials for treatment of pancreatic cancer in combination therapy with gemcitabine29-31,34-35. The modest anti-tumor effect of the antagonist may be due to several factors. First, many cancers involve multiple genetic mutations that are not easily treated by a single agent. There exist multiple redundancies, cross talk and possible compensatory mechanisms between signaling pathways. Consequently blocking one part of a pathway may not always provide enough improvement/antitumor activity that could be translated to improved survival outcome. Secondly, ongoing mutations in the primary molecular target of the drug may also result in drug resistance towards some of these therapies. Third, the presence of a constitutively active splice variant CCK2i4svR means that blocking CCK2R activity using an antagonist may be less effective.
The development of additional agents that selectively target CCK2R and that have the ability to deliver payloads (cytotoxic or diagnostic) would broaden the arsenal of agents that could be used in the treatment and diagnosis of tumors in which CCK2R is expressed. Such agents may also be used in the diagnosis and imaging of cancer in a subject.